The present invention relates to calenders in general, and more particularly to improvements in calenders of the type wherein the train of rolls comprises at least two superimposed rolls, wherein the lowermost roll has a hollow cylindrical member which surrounds a carrier mounted in bearings which are movable up and down along the roll stand, and wherein the carrier is held against angular movement in or with its bearings. Still more particularly, the invention relates to improvements in calenders wherein the cylindrical member of the lowermost roll is movable relative to its carrier by hydrostatic displacing means controllable by a regulating unit which causes the displacing means to compensate for flexure of the cylindrical member in actual use of the calender.
German Auslegeschrift No. 22 54 392 discloses a calender wherein the hollow cylindrical member of the lowermost roll cooperates with the roll which is disposed immediately thereabove and is mounted in a pair of bearings, the same as the carrier for the cylindrical member of the lowermost roll. The cylindrical member surrounds a displacing means including a row of cylinder and piston units which are staggered with reference to each other, as considered in the axial direction of the cylindrical member. The pressure of fluid in the cylinder chambers of such units must suffice to compensate for the weight of the cylindrical member, for the weight of any intermediate rolls if the train of rolls consists of three or more rolls, for any and all stresses which develop as a result of transport of a web of material to be treated through the calender, as well as to compensate for the tendency of certain portions of the cylindrical member to flex and to thereby change the profile of the nip between the lowermost roll and the roll immediately thereabove. Since the pressure of fluid in the cylinder chambers of the cylinder and piston units in the interior of the cylindrical member depends on a host of variable parameters, the regulation of such pressure presents many problems, especially since each and every change in stressing of the cylindrical member entails a change of fluid pressure in the aforementioned cylinder chambers because each such change in stress upon the cylindrical member can cause the cylindrical member to flex and to thus vary the width of the gap between its peripheral surface and the peripheral surface of the roll immediately above the lowermost roll. Each change of fluid pressure influences the load upon the cylindrical member, i.e., changes of fluid pressure in order to compensate for flexing of the cylindrical member can necessitate additional changes of fluid pressure in order to compensate for the changing load upon the cylindrical member as a result of those pressure changes which were carried out in order to prevent undue flexing of the cylindrical member. Moreover, the cylindrical members of presently known calender rolls which are constructed and assembled in the above outlined manner float on their carriers and, therefore, such cylindrical members exhibit a pronounced tendency to vibrate when the calender is in use.